The measurement of particulate matter in ambient air is important for a variety of reasons, the most important of which is related to health effects. Suspended particulate matter is known to produce a variety of deleterious health effects when inhaled. As a result, regulatory agencies around the world require monitoring of the levels of particulate matter. The levels are measured in terms of concentration, i.e. micrograms of particulate matter per cubic meter of air. Reference techniques for this measurement are presently defined in terms of a mass measurement utilizing a filter medium to capture the particulate matter and the total volume of air which has been filtered by the medium over a given period of time. There are various means available to unambiguously determine the flow rate through the filter over time (and hence the volume of air sampled), but surprisingly the mass measurement is not straightforward due to the complex nature of ambient particulate matter which results in unstable mass deposition on the filter.
This problem involving the measurement of particulate matter in ambient air is well-known. The uncertainty arises since the particulate mass used as a basis for mass concentration computations is defined as the mass captured on the filter media which is not necessarily the mass of the particles as they exist in the ambient air. Unlike measurements of major criteria gaseous pollutants, what is defined as particulate matter can change its mass as a result of loss or gain of volatile substances associated with the particulate matter and filter media. While gaseous pollutants exist as definable molecular species (SO.sub.2, O.sub.3, CO, etc.), particulate matter can be a combination of different substances with different volatilization rates, reactive, desorptive, absorptive, and adsorptive properties. In addition, the mass of particulate matter landing on the filter can be affected by the filter material itself, the particulate matter already collected on the filter, the face velocity through and pressure drop across the filter, as well as by the humidity, temperature and composition of the gas stream passing through the collection medium.
Both direct and indirect measurement techniques have been employed in an effort to quantify particulate matter mass. Each method which has been developed to date, however, has limitations in obtaining a measurement of the actual mass of particulate matter as it exists in its suspended form. Direct mass measurements as represented by weighing material captured on a substrate such as a filter are susceptible to instrument effects due, for example, to temperature or pressure changes, and to volatile component losses which are not easily quantifiable. Indirect methods such as light scattering measurements on the other hand are inherently inaccurate as there is no physical connection between other properties of particles and particle mass.
To compensate for instrument effects in direct mass measurements, a differential particulate mass measurement microbalance employing a pair of oscillating quartz crystal detectors has previously been proposed. In this earlier approach, a particle laden gas stream impacts upon the first detector and a particle free gas stream impacts the second detector. The second mass detector is used as a reference to cancel out detector instrument effects from a mass reading provided by the first detector. U.S. Pat. No. 5,571,945 discloses a similar measurement approach employing a pressure sensor to measure a pressure differential between a pair of particulate matter collectors; U.S. Pat. No. 5,349,844 discloses a similar approach for use with a filter that is caused to oscillate in a direction substantially perpendicular to a plane of the filter. However, volatilization losses are not accounted for in these earlier systems.
As a result of the above described difficulties, the current reference method in the United States is a method dependent technique which does not necessarily represent an accurate measure of particulate mass as it actually exists in its undisturbed state in the air. The reference method consists of filter equilibration under a defined range of temperature and humidity conditions, a pre-collection weighing of the filter, the installation of the filter in a manual sampler, the sampling of ambient air (for a 24-hour period), the removal of the filter from the sampling device, a post-collection conditioning under the same equilibration conditions as before, and finally a post-collection weighing. This methodology is intended to provide a consistent set of measurements between identical samples.
However, for the reasons stated above, results based on this method do not represent measurements to which an accuracy can be assigned, even loosely, i.e. to what accuracy is the particulate mass as it exists in the atmosphere measured by the mass determined from the filter? This is a serious problem, and one has to accept the fact that these measurements are only an indication of particulate levels. As a result, the current reference method represents simply a standardized procedure, and not a scientifically-based measurement standard for airborne particulate matter.
Volatile components are a confounding influence on these measurements. While the filter resides in the sampling hardware, important factors that influence the reactions taking place on the filter substrate, such as temperature and humidity, vary in an ill-defined manner. During sampling, the mass on and of the filter can increase dramatically during periods of decreasing temperature and increasing relative humidity (nighttime), and may experience substantial loss of semi-volatile materials when the temperature increases and humidity decreases (daytime). These same type of effects can be associated with air mass changes, and other meteorological events. Further, the collection filter may be exposed to widely varying hot or cold temperatures once sampling is complete and before it is removed from the sampler as well as during transportation to a laboratory for conditioning and weighing.
Not only does the mass of collected particulate matter and the filter change depending upon the conditions to which they are exposed, but the air stream through the filter creates a pressure differential across the filter which tends to strip off volatile components of the particulate matter. In effect, the interaction of the particles with the filter tends to modify the nature of the particulate matter as soon as it is collected, thereby affecting the accuracy of the desired measurement of the particulate matter as it is suspended in ambient air. As health concerns heighten, and measurement instrumentation becomes more sensitive, there is a trend towards measurement of even finer particulate matter, e.g. particles of 2.5 microns or less. With smaller particles, the impact of volatilization losses upon the mass measurement readings becomes even more pronounced.
A compelling need thus exists for a measurement instrument that can accurately measure the mass or concentration of particulate matter suspended in ambient air or other gaseous environments.